By the YELI Technical Team · Updated September 2026 · 8 min read
What Loaded Carbon Elution Actually Is
In a CIP/CIL plant, gold dissolves in cyanide solution and is adsorbed onto activated carbon as pulp moves through the tanks. For the carbon to keep working, that gold has to come off again — and that is what elution is for. It reverses adsorption: loaded carbon is screened to remove pulp, charged into an elution column, and contacted with hot eluant so the gold cyanide complex desorbs from the carbon surface and pore structure into solution.
The rich eluate goes to electrowinning, where the gold plates out for smelting and refining. The stripped carbon is cooled, regenerated in a kiln, and returned to the adsorption circuit to be loaded again. Elution sits between adsorption and refining, and it sets the grade and volume of solution your electrowinning and refinery have to handle.
Zadra vs AARL: The Two Workhorses, Compared Honestly
Two methods dominate gold carbon elution: Zadra and AARL (the Anglo American Research Laboratories method). Both do the same job, and both, run properly, strip into the mid-to-high 90s percent efficiency. The real differences are pressure, temperature, chemistry — and what each costs to build and run.
Which method belongs on your plant is an engineering decision. Plant scale, existing column design, and power availability all feed in. AARL buys speed and concentration at the price of pressure vessels and tighter operation; Zadra buys simplicity at the price of longer strips and leaner eluate.
- Zadra: atmospheric elution at roughly 90–95 °C, with a sodium hydroxide (NaOH) plus sodium cyanide (NaCN) eluant circulated through the column. Equipment is simpler, operation is forgiving, and it is the common choice on smaller and mid-size circuits.
- AARL: a pressurized column running at higher temperature, with a strong caustic pre-soak before stripping. It strips faster and produces higher peak eluate grades, which is why larger plants usually choose it.
The Levers That Decide How Well Elution Works
Strip performance is decided by a handful of levers. When strips run long, residuals climb, or eluate grades fall, work through these in order before touching the equipment.
None of these levers needs a laboratory to monitor. Temperature gauges, make-up solution checks, and a look at the bed during change-out tell you most of the story.
- Temperature: desorption is strongly temperature-driven. Hold strip temperature where the method specifies; a sagging steam supply shows up as slow strips first.
- Eluant strength: the NaOH and NaCN concentrations, and their ratio, set the driving force for desorption. Too weak, and gold comes off slowly; drift in make-up strength is a classic hidden cause of a slowing strip.
- Contact time and flow: eluant has to spend enough time against the carbon. Flow that is too fast short-circuits the bed; flow that is too slow starves column throughput.
- Column packing: fines in the bed cause channeling — eluant races down preferential paths and bypasses much of the carbon. Bed condition is a mechanical problem with a chemical result.
- Carbon condition: scaled, fouled, or soft carbon makes every lever above behave worse.
Acid Wash First: Remove the Scale Before You Strip
Most CIL circuits dose lime for pH control, and lime means calcium. Calcium carbonate precipitates onto the carbon, builds up inside the pore structure, and physically blocks the routes eluant must travel to reach the gold. A scaled bed strips slowly no matter how hot or strong your eluant is.
That is why loaded carbon is often acid-washed before elution. Washing with dilute hydrochloric acid (HCl) dissolves the calcium scale and reopens the pore area. In many plants the wash happens in the elution column itself: acid wash, rinse, then strip. The acid wash does not just speed up elution — it also removes other inorganic foulants before they can interfere with electrowinning or carry over into the kiln.
If strip times have been creeping up over weeks, pull a carbon sample and look for whitish scale. The acid wash is cheap; the strip time it saves is not.
Carbon Quality: Why Soft or Inconsistent Carbon Elutes Unpredictably
Elution assumes the carbon in the column is hard, clean, and uniform batch to batch. Real carbon is not always that. Soft carbon breaks down in screens, in the elution column, and again in the kiln, and every gram of fines is gold-bearing material that is hard to recover. Fines also pack into the bed and cause the channeling that makes strips unpredictable. Fouled carbon — carrying organics and inorganics that were never washed off — behaves differently from strip to strip. Consistent hardness and low fines matter in every leg of the cycle: screening, elution, and kiln.
That is why coconut shell carbon is the gold industry standard: hardness (low attrition means less gold lost in fines) plus a micropore structure that suits the gold cyanide complex. Palm kernel shell and other fruit-shell carbons are cheaper, but generally softer and less consistent, and some suppliers blend or substitute them. Check what you are actually buying. Price (FOB China, 2026 reference, iodine roughly 950–1100 band) runs about $3,200–3,800/t for gold-grade coconut shell against $1,200–1,800/t for coal-based material — and carbon that crumbles in your column is expensive at any price.
At our factory in Longyan, Fujian, China, we make only coconut shell gold-grade carbon and issue a COA with every batch (YS/T 3038—2020): 6×12 mesh (3×6 mm also available), iodine 1000–1300 mg/g, hardness ≥98% (ball-pan, ASTM D3802), moisture ≤5%, ash ≤5%. Compare that against any supplier's claims on our full specification sheet.
How to Sanity-Check Elution Performance on Your Plant
You do not need a laboratory program to see whether elution is earning its keep. For every strip, record the loaded carbon grade and the stripped carbon grade — the difference is what the column recovered. Track eluate grades through the strip and reconcile them against bullion produced and carbon moved. If stripped carbon grades start climbing toward fresh loaded values, walk the lever list: temperature, eluant strength, contact time, acid wash history, bed condition.
Done consistently, this gives you a strip-to-strip baseline, and the baseline is your fastest early warning. When a strip drifts, ask what changed — carbon batch, steam pressure, make-up solution, kiln settings — before touching the equipment.
Elution is the last place gold recovery can be lost, and it is also the easiest one to check. If you are sourcing or changing carbon, we ship gold-grade coconut carbon to West Africa via Dakar, Abidjan, and Tema — WhatsApp +86 136 0090 6330 or our contact page. For the full picture from adsorption to bullion, start with our complete guide to gold recovery from carbon.
Stripping Is Where Recovery Can Slip — Talk to Us
YELI Carbon is a coconut shell activated carbon manufacturer in Longyan, Fujian — 20+ years, gold-grade lines, hardness ≥98%, COA with every batch. If your strip performance is drifting, send us a sample of your current carbon and we will run a side-by-side test: iodine, hardness, gold adsorption. See the real gap before you change supplier.
Request a Sample TestFAQ: Gold Carbon Elution
Zadra runs at atmospheric pressure at roughly 90–95 °C with a sodium hydroxide plus sodium cyanide eluant — simpler equipment, easier operation, and common on smaller and mid-size circuits. AARL is pressurized and runs hotter, with a strong caustic pre-soak before stripping. It strips faster and produces higher peak eluate grades, which is why larger plants usually pick it. Method choice follows plant scale, column design, and available power.
There is no fixed cycle time. A strip is measured in hours, not minutes, and the total depends on method, column size, flow, eluant strength, temperature, and how much gold the carbon carries. AARL finishes faster than Zadra for the same column because it runs hotter under pressure. Treat cycle time as a tuning lever: too short, and residual gold climbs; unnecessarily long, and you waste steam and power.
Calcium carbonate scale — and other inorganics — builds up on carbon in CIL because lime is used for pH control. That scale blocks pores and slows gold desorption. Washing the loaded carbon with dilute hydrochloric acid before stripping dissolves the scale and reopens pore area, so eluant reaches the gold and the strip runs faster and more completely. A quick acid wash pays for itself in shorter strip cycles.
In a well-run circuit, strip efficiency lands in the mid-to-high 90s percent. Practically, stripped carbon should carry only a small residual fraction of its loaded grade — nowhere near the value of fresh loaded carbon. If residuals start climbing, check temperature, eluant strength, contact time, and whether scale fouling or soft, fine carbon is the culprit. Compare every strip against your own baseline rather than an industry rule of thumb.